Process control devices are typically used to control, measure, and/or perform other functions within a process such as opening or closing valves and measuring process parameters. For example, some process control devices may control the pressure of control fluid used to position a pneumatically-controlled device, such as a regulator. For example, an electro-pneumatic controller can be used to control a field device associated with the controller, which may be, for example, valves, valve positioners, switches, transmitters, and sensors (e.g., temperature, pressure, and flow rate sensors). In some forms, these controllers may control the pressure of control fluid into a pneumatic actuator of a process control valve or regulator to position the process control valve or regulator. For example, a valve may open or close in response to a control output received from a controller, or may transmit to a controller a measurement of a process parameter so that the controller can utilize the measurement as a control input. In some cases, the controllers may be used in hazardous environments that are susceptible to damage. Further, the controllers and regulators may use fluids that are flammable or explosive in nature. In such cases, explosion proof containers are used to contain explosions therein and to protect the installed environment as well as other control instruments to ensure proper operation.
Explosion resistant process control devices are capable of withstanding explosions and other shocks. When using explosion resistant process control devices, the installation site and/or technician typically must obtain clearance or a hot-work permit to perform work (e.g., maintenance, diagnostics, and/or routine checks) on the device or must alternatively decommission and relocate the instrument in order to modify its operation. This process can be both time-consuming and costly. Further, explosion resistant process control devices typically do not allow access to the components contained therein, as any breach of the container may impact the ability of the controller to withstand explosive forces. Accordingly, in order to communicate with the controllers (e.g., to send commands, modify inputs, and/or adjust variables); existing explosion resistant controllers have used a variety of technologies such as optical, and/or magnetic systems to communicate with components contained within the enclosure.
In environments where optical systems are used for communication with the controller, transmitter/receiver pairs are used to interface with the process control device in the explosion resistant container. Light from the transmitter reflects off of an input device (e.g., a user's finger) and the reflected light is received at the receiver signaling an input to the process control device. These systems are limited by both power constraints and the fact that only a discrete number of inputs may be available. As an example, some explosion resistant controllers are used in environments that have a 4-20 mA signal to power the device. Optical systems oftentimes require at least half of this available power for proper operation of systems. Thus, they generally cannot be used for more complex installations. Further, these systems are limited because, in some cases, each desired input of the instrument can require its own optical sensor, which, due to size and power constraints, can limit the number of total sensors that can be used within the enclosure. In addition, these systems can be unreliable due to the likelihood of non-transparent material such as dirt and/or grime depositing on enclosure surface. Similarly, in environments where magnetic systems are used, a physical button is actuated which creates or disrupts a magnetic field signaling an input. These interfaces are typically constrained by their size and thus may only have a limited number of discrete inputs.